The present invention relates to a method and apparatus for perforating a film web, and more particularly, a method and apparatus for perforating a film web which is wrapped on a load.
Stretch wrapping machines have been used to wrap a load with a web of material to contain the load. Exemplary stretch wrapping methods and apparatus are illustrated in U.S. Pat. Nos. 4,418,510, 4,317,322, and 4,302,920 to Lancaster et al. and assigned to Lantech Inc., which are incorporated herein by reference
Some wrapped products are required to be ventilated if they remain wrapped over an extended period of time. As such, either a web of netting has been used, or a web of plastic film with apertures has been used.
The use of netting has several drawbacks. It has a high material cost and the apertures in the netting are available only in certain standard sizes.
The alternative of using plastic film with apertures also has several drawbacks. Apertures which are mechanically created by punching or piercing results in pieces of the plastic film being separated from the web and becoming a foreign substance in the product being wrapped. This is undesirable especially when the product is a consumable such as food or tobacco.
In addition, mechanically creating apertures in film webs conventionally involves apparatus that quickly dull and clog. As a result, such apparatus needs to be cleaned or replaced on a regular basis and thus causes expense and interruption in use of the wrapping equipment.
Furthermore, the apparatus used for making apertures in the film web do not offer the desired flexibility in controlling the size and location of the apertures in the film web.
Also, it has been difficult to control perforation devices to create holes that are large enough to provide air flow but not so large as to cause the film web to lose holding strength. The tearing action of conventional perforation devices sets up weakened areas in the film which promote undesired tearing and loss of film strength.
Further, the requirement that conventional and mechanical perforation devices contact the film increases the sensitivity of the film to tearing and jamming while it is being handled and limits the conditions under which the film can be placed while being perforated, such as the amount of stretch and the variations in stretch of the film while being perforated.
In addition to the need to properly ventilate wrapped products, there is a need to provide wrapped products with a line of weakness or a tear strip in the wrapping material in a reliable inobtrusive manner to facilitate the removal of the wrapping material at a later time.
It is an additional object of the present invention to provide an apparatus for opening and facilitating the removal of the wrapping material when the product is to be used by formation of a line of weakness or tear strip, while avoiding the drawbacks of complex, expensive, and time consuming arrangements, and also to prevent the wrapped product from being damaged in providing such an opening arrangement.
Accordingly, it is an object of the present invention to provide an apparatus for perforating film which does not cause pieces of the plastic film to become separated from the web and become disposed as a foreign substance in the product being wrapped.
It is also an oblect of the present invention to provide an apparatus for perforating film which does not become dull or clogged and neither needs to be cleaned and replaced on a regular basis nor causes expense and interruption in the wrapping or perforation process.
It is another object of the present invention to provide an apparatus for perforating film which offers the flexibility in controlling the size and location of the apertures in the film web.
It is a further object of the present invention to provide an apparatus for perforating film which creates apertures that are large enough to provide air flow but not so large as to cause the film web to lose holding strength.
It is also an object of the present invention to provide an apparatus for perforating film which does not contact the film and which does not limit the conditions under which the film can be placed while being perforated, such as the amount of stretch and the variations in stretch of the film while being perforated.
It is an additional object of the present invention to provide an apparatus which forms a line of weakness or tear strip in the wrapping material in a reliable inobtrusive manner to facilitate the removal of the wrapping material at a later time.
Additional objects and advantages of the invention will be set forth in the description which follows and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.